Wireless transmitters often require a variety of waveforms over multiple frequency channels, such as continuous wave, frequency hopping, 16QAM or CDMA etc. The availability of output power, the linearity and bandwidth of the power amplifier and the power efficiency of the transmitter under those various modulation formats are all desired. One well-known problem in maintaining high transmitter efficiency with linear power amplifiers is that the efficiency of conventional microwave power amplifiers is discounted when the output power is backed off because of amplitude fluctuations caused by non-constant envelope modulations. In the past, a number of transmitter efficiency enhancement techniques have been proposed to alleviate this problem [see, e.g., F. H. Raab, P. Asbeck, S. Cripps, P. B. Kennington, Z. B. Popovic, N. Phthercary, J. F. Sevic and N. O. Sokal, “Power Amplifiers and Transmitters for RF and Microwave,” IEEE Trans. Microwave Theory & Tech., vol. 50, pp. 814-826, March 2002, and S. C. Cripps, RF Power Amplifiers for Wireless Communications. Norwood, Mass.: Artech House, 1999, both of which are incorporated herein by reference].
Among these techniques, the most promising one is called the load modulation technique. It is based on the simple fact that the power efficiency can be restored when the load impedance is adversely changed according to output power fluctuations. This is to keep the power amplifier devices always operating in close to saturation mode. However, to dynamically adjust the load impedance in RF power amplifiers is not an easy task given the high power conditions and the low loss requirements at the amplifier output. The load modulation itself, if not well controlled, can introduce extra distortions that can affect the linearity. Examples of typical load modulation techniques include the Doherty amplifier [see, e.g., F. H. Raab, “Efficiency of Doherty RF power-amplifier systems,” IEEE Trans. Broadcast., vol. BC-33, no. 3, pp. 77-83, September 1987, incorporated herein by reference] and the Chireix's outphasing amplifier [see, e.g., F. H. Raab, “Efficiency of outphasing RF power amplifier systems”, IEEE Transactions on Communications, Vol. COM-33, No. 10, pp. 1094-1099, October 1985, and A. Birafane and A. B. Kouki, “On the linearity and efficiency of outphasing microwave amplifiers”, IEEE Trans. on Microwave Theory & Tech., Vol. 52, No. 7, pp. 1702-1708, July 2004, both incorporated herein by reference]. Both of these techniques use the load pulling phenomenon that occurs when two transistor outputs are combined to realize load modulations. The load modulations are controlled through different gate biases on the two transistors in the Doherty amplifier and through phase modulations in Chireix's outphasing amplifier.
However, the potential efficiency improvement due to load modulations in either configuration is not optimal when the output power fluctuates over a wide range. In addition, as the load modulations are realized in an analogue fashion, they may incur significant amounts of nonlinear distortions in the amplification process. Pre-distortion techniques are usually required in order to maintain the linearity, which increases the complexity and limits the bandwidth performance of the system. Alternative approaches to load modulation for efficiency enhancement are drain modulation techniques such as Kahn techniques or Envelope Elimination and Restoration (EER) techniques [see, e.g., F. H. Raab, B. E. Sigmon and R. G. Myers, “L-Band transmitter using Kahn EER technique” IEEE Trans. on Microwave Theory & Tech., Vol. 46, No. 12, pp. 2220-2225, December 1998, incorporated herein by reference], which rely on the drain voltage modulation to generate non-constant envelopes at the output and to save DC power consumption. Though the EER technique promises optimal efficiency over any power levels for ideal devices, it requires a linear switching power supply with good efficiency and large output current. Such switching power regulators usually have low switching rates and are not available for broad band modulations.
What is needed is an improved method and apparatus for microwave power amplification that will ideally maintain maximum efficiency over a wide range of power output levels in the microwave spectrum. The embodiments of the present disclosure answer these and other needs.